Such developing apparatuses can be classified into apparatuses which use a two-component developing agent consisting of toner and a carrier and apparatuses which use a one-component developing agent consisting only of magnetic toner. With recent technical advances in this field, a developing apparatus which uses a one-component developing agent consisting only of nonmagnetic toner which can resolve defects of the one-component developing agent consisting only of magnetic toner has been developed. However, such a developing apparatus has a big problem in that it is difficult to stably form a uniform thin toner layer on a surface of a movable developing agent carrier, thereby preventing the practical use of this apparatus.
In this manner, the present inventors have invented a developing apparatus shown in FIG. 1 (Japanese Patent Application No. 57-155934), and they have succeeded in forming a thin layer of nonmagnetic toner. In this invention, as shown in FIGS. 1 and 2, a rubber blade 2 as a flexible coating member is brought into surface contact with a developing roll 1 as a movable developing agent carrier except for a free end portion of the rubber blade--i.e., an edge portion positioned at a downstream side along the flow of the developing agent. Nonmagnetic toner 4 is fed from, for example, a toner holder 3 to a surface of the developing roll 1. The toner 4 is uniformly coated by the rubber blade 2 on the surface of the developing roll 1, thereby forming a thin toner layer. This thin layer is opposed to a photosensitive drum 5 as a latent image carrier, thereby developing a latent image thereon.
According to the developing apparatus described above, the contact area between the surface of the movable developing agent carrier and the flexible coating member can be set to be large. In addition, no edge portion of the flexible coating member is brought into contact with the surface of the developing agent carrier, thereby preventing a pressing force from acting on the surface thereof. As a result, nonuniformity caused by variations in setting conditions, mechanical precision, wear or the like can be moderated, thereby forming a satisfactorily uniform toner layer. Moreover, since the contact area between the surface of the carrier and the coating member is large, the developing agent is subjected to friction for a sufficiently long period of time under the pressing force when it passes through this contact portion. As a result, the developing agent can be uniformly and sufficiently charged by friction. Therefore, since the developing agent having a sufficient electric charge can be formed into a uniform thin layer, a latent image can be satisfactorily developed.
However, the present inventors found by experiment that the above-mentioned developing apparatus has the following problems.
(1) In order to form a uniform thin toner layer, the pressing force on the rubber blade must be set to be larger than a predetermined value. Therefore, the toner layer formed on the surface of the developing roll becomes extremely thin. Microscopically, toner particles or toner aggregate is sparsely applied on the surface of the developing roll. As a result, when the toner layer is arranged to oppose a latent image carrier and then noncontact development is performed to form a developed image, the thus developed image cannot have a sufficient copy density.
(2) The nonmagnetic toner used in this apparatus must have a considerable flowability. When toner having a poor flowability is used, a toner path under pressure of the rubber blade is clogged by the toner aggregate, and the toner cannot pass this portion. For this reason, stripes are undesirably formed in the toner layer on the surface of the developing roll.
(3) When a foreign material is mixed in the developing agent particles, the foreign material clogs the toner path under pressure of the rubber blade, and the toner cannot pass this clogged portion. For this reason, stripes are undesirably formed in the toner layer on the surface of the developing roll.
The above problems (2) and (3) are mainly caused by undesirable slippage between the toner or the toner layer and the surface of the developing roll under the pressing force of the flexible coating member. This will be described with reference to FIG. 3. The toner 4 or a toner aggregate 6 is brought under the influence of the pressing force of the rubber blade 2 upon rotation of the developing roll 1 and is subjected to a blocking force F1 of the blade 2 and a feeding force F2 of the developing roll 1. Formation performance of the toner layer is determined by the shear property of the toner aggregate 6 under the forces F1 and F2. When the maximum static friction coefficient between the toner 4 and the developing roll 1 is small, the toner aggregate 6 begins to slip on the surface of the developing roll 1 before it is sheared, and it collects under the rubber blade 2. The passage of further toner particles is prevented by the collected toner aggregate. As a result, stripes having no toner particles are formed on the surface of the developing roll 1. It should be noted that, even if the part designated by the reference numeral 6 in FIG. 3 is not the toner aggregate but is a foreign material, stripes are formed in the toner layer in the same process.
The problem (1) is caused by the following behavior of the toner particles. In the toner particles collected due to the slippage, the toner particles which are in contact with the surface of the developing roll gradually increase their frictional charge, and an electrostatic attracting force with respect to the developing roll is also increased. When the attracting force is increased, the dynamic frictional force between the toner particles and the developing roll is also increased. When the frictional force reaches a predetermined value, the toner aggregate is sheared. The thus sheared toner is fed by the developing roll. In this manner, since the toner aggregate is intermittently sheared, the toner layer having a low copy density is sparsely formed on the surface of the developing roll, resulting in a low copy density of the developed image.